Articles techniques et présentations

The transition from fossil fuels to renewable energies requires finding new sources of energy. It turns out that the hydrothermal potential of the lakes would cover a significant part of these renewable energy needs. The existing Thermal Lacustrine Networks (TLNs) are efficient in summer allowing the building air conditioning by « free-cooling » (without heat pumps) thanks to the cold water pumped from the bottom of the lakes. However, in winter, the heating of the buildings is 4 to 5 times less efficient due the use of heat pumps and because of the lower temperature differential of the water round-trip of the lakes. The ULISSE project, supported by the Swiss Federal Office of Energy, aims to build an underwater tank made of a semi-rigid envelope that could be filled with the warm water pumped from the surface of the lakes, heated by the sun during the hot season and to keep that water as warm as possible thanks to the thermal insulation properties of the envelope until the cold season. In winter, that water would be pumped back from the tank. The higher temperature (versus normally at the lake bottom) of the pumped water would allow to reduce by 95% the pumping energy and provide an important heat source for the heat pumps (twice their efficiency) of the said TLNs (Fig. 1). This concept reproduced over the fifteen largest lakes in Switzerland would allow to economize 3 TWh of electricity consumption during the winter.

In this paper we present a COMSOL Multiphysics® simulation model of a reduced size ULISSE mock-up (Fig. 2). The mock-up is immersed in a container, playing the role of the lake. Both are filled with water at the room temperature. Then starts a cycle imitating the change of seasons. Hot water at 42 0C is injected by a pump in the ULISSE mock-up chasing the initial water. After a period of rest, the water is pumped back. The recovered energy is calculated. Real-time measurements are made by temperature sensors and water flow sensors allowing comparisons with the simulation results.

Taking profit of the symmetries of the system, a quarter of the real geometry has been modeled. The model is using the Laminar Flow interface of the CFD Module, with the Boussinesq approximation for gravity effects, and the Heat Transfer in Fluids and Solids interface of the Heat Transfer Module. The physics are coupled through the velocity and the temperature field. The simulations allow to understand the fluid flow and the temperature distribution (Fig. 3). The computation of the energy balance and the temperature evolution at different points was compared with the experimental results. The results of the simulations with the COMSOL Multiphysics® software are in good agreement with experimental observations validating the proof of concept.